Regiospecificity and Stereochemistry

Zimmerman and Baum observed only the formation of (9) from 

This regioselective example of a di-n--methane photorearrangement is of particular importance because in this case both it moieties have very similar singlet energy levels. This suggests that excitation localization does not control the reaction. 

The di-7r methane reaction is not only r oselective but is also stereoselective, as indicated by the following examples  

Upon low conversion direct photolysis the cis isomer (10) gave only the ds isomer (12) and the irons isomer (11) gave only the irons isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-irons isomerization. Thus the diir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-irarts isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specificity and stereospecificity in di-n-methane photoreactions are as follows  

Substitution at the Central sp Carbon Atom and Di-ir-Methane Reactivity 

---